Genetics Unit SBI3U
Pedigrees
Pedigrees can be used to track inheritance of certain traits throughout a family linage. Scientists use various symbols and lines to represent gender and whether or not the individual is a carrier of the trait being tracked.
The symbol of a square always represents a male family member. Typically a non-affected male is represented by an unshaded square, while an affected male is represented by a shaded square. Females however are represented by circles. A non-shaded circle represents an unaffected female, while a shaded circle represents and affected female.
There are typically 4 different line that represent the relationship between individuals on a pedigree. A straight line through the middle of two shapes means that those individuals are married, a line connected in a box like way at the top of the two shapes indicates that the individuals are siblings, two shapes which are connected in a triangle shaped line with another one through the middle of the triangle represents identical twins and two shaped that re connected in a triangle shaped pattern with a line at the top represents fraternal twins.
By reading a pedigree you can also diffareate between the different generations. Each generation is represented by a royal numeral on the outside of the pedigree. While the order in which the individuals are born are represented with normal numbers. You read the pedigree from left-right, which means the person farthest left is the oldest while the person farthest right is the youngest.
Multi-trait inheritance
Mendel's first experiment focused on only one trait. A little while later Mendel realized that organisms could posse heterozygous traits and that inheritance is dependant on different independent characteristics.
From this Mendel came up with the law of independent assortment. The law of independent assortment states that "If genes are located on separate chromosomes, they will be inherited independently of one another". This means that a heterozygous individual for two characteristics produces four possible gametes. To figure out the phenotype of the trait your looking for you can use a 4x4 punnet square.
Multi-trait inheritance also applies to something known as the product law. The product law states that "The probability of two independent random events both occurring at the same time can be calculated based on the probability of inheriting each trait individually"
Mendelian Genetics
Gregor Mendel is known as the father of genetics, he determined the general principles of inheritance by breeding pea plants. Mendel chose to breed pea plants because they grew in multiples, they were cheap and easy to grow, they could be cross pollinated easily, etc. In Mendel's first experiment he conducted a monohybrid cross (single trait inheritance). After Mendel finished his first experiment he was able to determine that there are dominant and recessive traits, and that these traits were passed on by heredity units that could not be seen. This is also known as the law of segregation. The law of segregation states that for each characteristics, an organism carries two factors (genes): one from each parent and that organisms donate only one copy of each gene in their gametes.
These conclusions lead us to what are known as genes and alleles. What Mendel considered factors are now called genes and the hidden factors that were expressed are alleles. Alleles can be shown by upper case and lower case letters, upper case letters represent dominant alleles and lower case letters represent recessive alleles.
When attempting to figure out the phenotype of a specific trait we can use something called a punnet square. A punnet square is a square that can help us determine the probability of offspring possessing a specific trait if we know both the parents genotypes. Typically you put the parents genotypes above the square then divide it into four boxes. You then add the letter that is above that box and beside the box into the box, you put the domaint (capital letter) first then the ressivive (lower-case letter), you go around the square until all the boxes are filled out and then you can read the boxes and dertimine the apperance of the trait using the key. You can also use the info in the punnet square to create genotypic and phenotypic ratios.
From Mendel's experiment he conclude that the physical appearance of an organism is called the phenotype and the genetic make up is known as the genotype
Mitosis
There are 5 different stages of cell division (+ cytokinesis) when dealing with somatic cells. The different stages can be remembered by and acryonom of PMAT. PMAT stands for prophase, metaphase, anaphase and telophase. In prophase the chromatin condense into chromosomes, homogenous ones pair and the nuclear envelope disappears. In metaphase the chromosomes align at the metaplate and the spindle fibres attach. In anaphase the homogenous chromosomes are pulled to opposite ends of the cell. In telophase the chromosomes unravel into chromatin and the nuclear envelope reappears. And in cytokinesis the cytoplasm divides and the cell becomes 2.
Cell Division
Cell's divide in order to reproduce, they are unicellular so they need to reproduce to create offspring and increase the number of them. Cells may also divide to grow and repair and produce gametes (sex cells). There are 3 major events in the lifecycle of a cell; Replication (copy DNA and prepare for separation), Mitosis (actual division of the cell) and Cytokinesis (when the cytoplasm divides and one cell officially becomes 2).
There parts within the cell in which genetic info is organized. With in the cell there are chromosomes which are a piece of DNA that is filled with many genes. Genes are sections of DNA which provide the proteins with instructions. Alleles are versions of the same genes. And while in the process of division we find something called homologous chromosomes, which are two chromosomes that carry the same set of genes and provide proteins with the same set of instructions.
Cells may also have different numbers of chromosomes within them. A cell in which there are 23 chromosomes is known as a haploid cell (represented by n), an example of a haploid cell are gametes (sex cells). A cell which there are 46 chromosomes is known as a diploid cell (represented by 2n) these cells are typically somatic cells (body cells).
Structure of DNA
DNA stands for deoxyribonucleic acid, this molecule stores transmits genetic information from parent to offspring. Up until the 1930's it was unknown where DNA was acutely stored, until Hammering discovered it was stored in the nucleus while experimenting with algae. A DNA molecule is shaped in a double helix shape (a ladder or spiral stair case). It is formed with 4 nitrogenous bases, these bases are connected in pairs, Adenine, Thymine, Cytosine and Guanine. The pairs are the following; Adenine and Thymine - Cytosine and Guanine. What bases are paired together can be remember by a simple acronym, apple in the tree followed by car in the garage (A,T - C,G). The discovery of the base pairs and the fact that DNA is made up of two separate strands was discovered by Watson and Crick. Adenine and thymine are held together by 2 hydrogen bonds, while guanine and thymine are held together by 3 hydrogen bonds. A DNA molecule also consists of ribose sugars and phosphate molecules.
Sex linked Inheritance
Within the body there are sets of certain chromosomes that determine our biological sex. For men that is an x and a y chromosome, while for women that is two x chromosomes. These chromosomes which we call sex chromosomes can be used to pass certain traits or mutations down to the offspring.
Traits that are found on the X - chromosome are X linked, while traits found on the Y - chromosomes are Y linked. Most traits that are passed down from the sex chromosomes are mostly passed down from the X chromosome due to it being larger in length and having more genes, some of which can't be found on the Y chromosome.
Most sex linked traits are recessive. So when writing genotypes for sex linked traits we have a capital letter, which is either X or Y and a subscript which is a lower case letter. for recessive traits and upper case letter for dominant traits. The letters at hand are what will determine whether or not the offspring has the trait or is just a carrier.
Variations in Heredity
There are 4 main types of heredity, Median genetics, incomplete dominance, codominance, and blood types.
Incomplete dominance is when neither of the two alleles present in the parents are expressed instead they are blend together to create an entirely new phenotype. (Ex. white and red flowers when crossed produce pink flowers)
Codominace is when both alleles inherited from the parents are expressed in the offspring. Codominace is basically when both traits that could be expressed are mixed within the offspring. (Ex. Black and white cat produce offspring that are a mixed of black and white - spotted cats)
Blood types are they're own type of dominance within heredity. Blood types follow a set of "rules", the rules are as follows; A and B are dominant to O, when A and B are crossed together they can produce a blood type which is codominant and O is always recessive.
Meiosis
When dealing with gametes there is 10 different stages (+ 2 cytokinesis stages - 14 in total). These stages can be remember by the same acryonom PMAT but it must be repeated twice so the acryonom would be P1M1A1T1 and cytokinesis1 - P2M2A2T2 and cytokinesis 2. In prophase 1 the chromatin condense into chromosomes, homogenous ones pair and the nuclear envelope disappears (this is when crossing over would occur). In metaphase 1 the chromosomes align at the metaplate and the spindle fibres attach. In anaphase 1 the homogenous chromosomes are pulled to opposite ends of the cell. In telophase 1 the nuclear envelope reappears. And in cytokinesis 1 the cytoplasm divides and the cell becomes 2. Then the process starts all over again. In prophase 2 the homogenous ones pair and the nuclear envelope disappears. In metaphase 2 the chromosomes align at the metaplate and the spindle fibres attach. In anaphase 2 the homogenous chromosomes are pulled to opposite ends of the cell. In telophase 2 the nuclear envelope reappears. And in cytokinesis 2 the cytoplasm divides and the two cell's becomes four.
Meiosis results in the formation of sperm also known as spermatogenesis. This process occurs in the testes and usually occurs from birth until death. When meiosis occurs it results in 4 equal spermatids.
Meiosis also results in the formation of eggs (ova) also known as oogenesis. This process occurs in the ovaries and will occur from puberty until menopause. When ovulation occurs 1 mature ovum and 3 polar bodies will be produced. This process of releasing eggs is caused by the hormone FSH being released during puberty.
The chromosomal mutation known as non-disjunction occurs during meiosis and is caused by the chromosomes not separating correctly. This can result in a trisomy (three chromosomes are present) or monosomy (only one chromosome is present).
Genetic Mutations
There are many reasons why genetic mutations can happen. Sometimes they happen due to having the wrong nucleotide or having too many or too little in a sequence. Normally a DNA molecule will detect theses errors and correct them but sometimes they go unnoticed and they then become permeant mutations. Mutations can be caused by environmental or chemical factors and these are known as induced mutations or they can be caused by an error during cell division which is called a spontaneous mutation. Mutations can be harmful, beneficial or natural. Mutations can occur in the somatic cells (body cells) or the gametes (sex cells). A mutation in the somatic cell will not be passed on the any offspring while mutations in the gametes are passed on to the offspring. There are two main categories in which we classify mutations;
2. Chromosomal mutations - these mutations are caused by errors within the chromosome. There are 4 different types of chromosomal mutations. This type of mutation effects a large part or all of a chromosome. The mutations are non-disjunction, large scale deletions, insertions and inversions.
1. Point mutations - consist of one of three different issues either base pair substations (bases aren't paired correctly, Ex. ATCGATTG - instead of ATCGATCG), insertion (when a nucleotide is added to a sequence) or deletion (when a nucleotide is missing from a sequence).
Bio Ethics
Bio ethics is the study of ethics with regards to medical and health sciences. Bio ethics is a very divided topic and be argued for use in a court of law or in a medical research lab. Bio ethics can detrimental wether or not a DNA sample sourced at a crime scene can be used in a murder trial or wether or not left over embryos can have research done on them. Bio ethics also relates to the topic of genetic testing and mutations which will further discussed later.
